1. Field of the Invention
The present invention relates to an oscillator with a resonant circuit of at least one inductance device and at least one tunable capacitance, wherein the tunable capacitance is implemented through diffusion capacitances of at least one current-carrying transistor.
2. Description of the Background Art
Diffusion capacitances arise when pn junctions are operated in the forward-biased direction. The diffusion capacitance results from the fact that, in forward-biased operation, the pn junction is flooded with charge carriers. This has the result that majority charge carriers migrate into the region of opposite doping. They remain there as minority carriers for a certain period of time before recombining. But until recombination, they travel further, traversing a recombination distance, resulting in a spatial separation of charge. A pn junction operated in the forward-biased direction with charge carriers stored in this manner therefore represents a capacitance.
The diffusion capacitance, which arises only with current flow, is to be distinguished from depletion layer capacitance, which is also formed at a reverse-biased pn junction due to static space charge. Depletion layer capacitances depend on an applied reverse voltage and the design of the diode, hence the depletion layer area, the semiconductor material and the doping. The capacitances are typically in the picofarad range, and vary nonlinearly as a factor UR−0.5 of the applied voltage UR.
In general, non-parallel-connected depletion layer capacitances, which is to say variable-capacitance diodes operated in the reverse-bias direction or specially grown varactor diodes, have been used to date for tuning resonant circuits. Variable-capacitance diodes are diodes in which the voltage dependence of the depletion layer capacitance is intentionally exploited. They embody variable, voltage-controlled capacitances. The basic material nowadays is primarily silicon, with gallium arsenide also being used for special applications. The depletion layer capacitance forms a component of the resonant circuit capacitance, which can be varied by a control voltage. Minimum capacitance is achieved at the largest reverse voltage. Maximum capacitance is achieved at the smallest reverse voltage. Varactor diodes are in principle high-power variable-capacitance diodes. The distortions arising in a resonant circuit due to the nonlinear capacitance curve c=f(U) during modulation with relatively large HF voltages are intentionally exploited here. The resulting harmonics are filtered out, and frequency multiplier circuits are designed in this way.
Because of their relatively low capacitance, diodes have a large area. Consequently, they use a great deal of chip area, and capacitive coupling through the substrate becomes increasingly critical as the size of the individual components increases. The size of the tuning range is proportional to the area, so the danger of coupling grows with the size of the tuning range. The substrate becomes ever more capacitive with increasing frequency, so coupling also becomes more likely with increasing frequency.
In order to avoid these disadvantages, diffusion capacitances are also used to tune resonant circuits. With identical components, the diffusion capacitance is significantly larger in numeric terms than the depletion layer capacitance, and is on the order of magnitude of a few hundred picofarads to a few hundred nanofarads.
Thus, the use of diffusion capacitances permits a reduction in the chip area required. An oscillator of the aforementioned type that operates with diffusion capacitances is known from U.S. Pat. No. 6,114,919. This document discloses an LC network of an inductance device and a resonant circuit capacitance, which is determined by the sum of the capacitances between a certain node (node 1) of the circuit and a reference potential (ground). This sum includes the diffusion capacitances of a transistor pair wired as differential amplifiers and the capacitance of a control transistor in an amplifying branch of the circuit. A change in the control current through the control transistor thus inevitably results in a change in the current through the transistors of the differential amplifier, displacing the operating point of these transistors. As a result, undesirable saturation effects, an undesirable change in the amplitude of oscillation, and a modulation of the noise properties of the resonant circuit can occur.